I. Field of the Invention
This invention relates generally to a dispatch system and, more particularly, to access regulation in a dispatch system.
II. Description of the Related Art
In a wireless telephone communication system, many users communicate over a wireless channel to connect to other wireless and wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307 issued Feb. 13, 1990 to K. Gilhousen et al., entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated herein by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters, airborne repeaters, or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times permitting an increase in system user capacity.
In the CDMA cellular system, each base station transceiver subsystem provides coverage to a limited geographic area and links the remote units in its coverage area through a system switch to the public switched telephone network (PSTN). When a remote unit moves to the coverage area of a new base station transceiver subsystem, the routing of the remote unit's call is transferred to the new base station transceiver subsystem. The base station-to-remote unit signal transmission path is referred to as the forward link and the remote unit-to-base station signal transmission path is referred to as the reverse link.
In an exemplary CDMA system, each base station transceiver subsystem transmits a pilot signal having a common pseudorandom noise (PN) spreading code that is offset in code phase from the pilot signal of other base station transceiver subsystems. During system operation, the remote unit is provided with a list of code phase offsets corresponding to neighboring base station transceiver subsystems surrounding the base station transceiver subsystem through which communication is established. The remote unit is equipped with a searching element with which it tracks the signal strength of the pilot signal from a group of base station transceiver subsystems including the neighboring base station transceiver subsystems.
A method and system for providing communication with a remote unit through more than one base station transceiver subsystem during the handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATION SYSTEM," issued Nov. 30, 1993, assigned to the assignee of the present invention. Using this system, communication between the remote unit and the end user is uninterrupted by the eventual handoff from an original base station transceiver subsystem to a subsequent base station transceiver subsystem. This type of handoff may be considered a "soft" handoff in that communication with the subsequent base station transceiver subsystem is established before communication with the original base station transceiver subsystem is terminated. When the remote unit is in communication with two base station transceiver subsystems, the remote unit combines the signals received from each base station transceiver subsystem in the same manner that multipath signals from a common base station transceiver subsystem are combined.
In a typical macrocellular system, a system controller may be employed to create a single signal for the end user from the signals received by each base station transceiver subsystem. Within each base station transceiver subsystem, signals received from a common remote unit may be combined before they are decoded and thus take full advantage of the multiple signals received. The decoded result from each base station transceiver subsystem is provided to the system controller. Once a signal has been decoded it cannot be `combined` with other signals. Thus the system controller must select between the plurality of decoded signals produced by each base station transceiver subsystem with which communication is established by a single remote unit. The most advantageous decoded signal is selected from the set of signals from the base station transceiver subsystems and the unchosen signals are simply discarded.
Because the remote unit is communicating with the end user through at least one base station transceiver subsystem at all times throughout the soft handoff process, no interruption in communication occurs between the remote unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" technique over the conventional "break before make" technique employed in other cellular communication systems.
In a wireless telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmission power of each remote unit is controlled such that each transmitted signal arrives at the base station transceiver subsystem receiver at the same level. In an actual system, each remote unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a remote unit arrives at the base station transceiver subsystem receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other remote units. On the other hand, if the remote unit transmitted signal is at a power level that is too high when received at the base station transceiver subsystem, communication with this particular remote unit is acceptable but this high power signal acts as interference to other remote units. This interference may adversely affect communications with other remote units.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each remote unit within the coverage area of a base station transceiver subsystem is controlled by the base station transceiver subsystem to produce the same nominal received signal power at the base station transceiver subsystem. In the ideal case, the total signal power received at the base station transceiver subsystem is equal to the nominal power received from each remote unit multiplied by the number of remote units transmitting within the coverage area of the base station transceiver subsystem plus the power received at the base station transceiver subsystem from remote units in the coverage area of neighboring base station transceiver subsystems.
It is also desirable to control the relative power used in each data signal transmitted by the base station transceiver subsystem in response to control information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward channel link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. An example of such a location is a point where the path loss to one or two neighboring base station transceiver subsystems is nearly the same as the path loss to the base station transceiver subsystem communicating with the remote unit. In such a location, the total interference would be increased by three times over the interference seen by a remote unit at a point relatively close to its base station transceiver subsystem. In addition, the interference coming from the neighboring base station transceiver subsystems does not fade in unison with the signal from the active base station transceiver subsystem as would be the case for interference coming from the active base station transceiver subsystem. A remote unit in such a situation may require 3 to 4 dB additional signal power from the active base station transceiver subsystem to achieve adequate performance.
At other times, the remote unit may be located where the signal-to-interference ratio is unusually good. In such a case, the base station transceiver subsystem could transmit the desired signal using a lower than normal transmitter power, reducing interference to other signals being transmitted by the system.
To achieve the above objectives, a signal-to-interference measurement capability can be provided within the remote unit receiver. This measurement is performed by comparing the power of the desired signal to the total interference and noise power. If the measured ratio is less than a predetermined value, the remote transmits a request to the base station transceiver subsystem for additional power on the forward link signal. If the ratio exceeds the predetermined value, the remote unit transmits a request for power reduction. One method by which the remote unit receiver can monitor signal-to-interference ratios is by monitoring the frame error rate (FER) of the resulting signal. Another way is by measuring the number of erasures received.
The base station transceiver subsystem receives the power adjustment requests from each remote unit and responds by adjusting the power allocated to the corresponding forward link signal by a predetermined amount. The adjustment is typically small, such as on the order of 0.5 to 1.0 dB, or around 12%. The rate of change of power may be somewhat slower than that used for the reverse link, perhaps once per second. In the preferred embodiment, the dynamic range of the adjustment is typically limited such as from 4 dB less than nominal to about 6 dB greater than nominal transmit power.
The base station transceiver subsystem should also consider the power demands being made by other remote units in deciding whether to comply with the requests of any particular remote unit. For example, if the base station transceiver subsystem is loaded to capacity, requests for additional power may be granted, but only by 6% or less, instead of the normal 12%. In this regime, a request for a reduction in power would still be granted at the normal 12% change.
When the original cellular telephone licenses were issued by the government, one of the restrictions on use of the spectrum was that the carriers could not provide dispatching system services. However, because of the great advantages of the CDMA system and the inherent expense and problems of deployment and maintenance of private dispatch systems, the government is re-examining this issue. The government itself would benefit greatly from such services.
Whereas typical wireless and wireline telephone service provides point-to-point service, dispatching services provide one-to-many service. Common usage of dispatch services are local police radio systems, taxicab dispatch systems, Federal Bureau of Intelligence and secret service operations, and general military communication systems.
The basic model of a dispatch system consists of a broadcast net of users. Each broadcast net user monitors a common broadcast forward link signal. If a net user wishes to talk, he presses a push-to-talk (PTT) button. Typically the talking user's voice is routed from the reverse link over the broadcast forward link. Ideally the dispatch system allows landline and wireless access to the system.
When a remote unit which is part of a dispatch system presses the push-to-talk button, he would like to immediately begin speaking. However in conventional wireless systems, a perceptible amount of time is necessary to establish a link before the user may begin speaking. The present invention is an efficient solution to system access. The present invention also provides a means to regulate and protect system access in dispatch system.